With its large energy band gap and other unique physical properties, diamond is regarded as a desirable material for many engineering applications including wear-resistant tool coatings, optical windows for visible and infrared transmission, abrasives, and particularly high temperature electronic devices. Diamond can be used as a high-grade, radiation resistant, high-temperature semiconductor with potential application in many commercial, military, and aerospace technologies. Thus, there is considerable interest in finding and improving techniques for synthesizing diamond films for these and other applications.
Various techniques for forming diamond films have been proposed. For example, U.S. Pat. No. 4,915,977 to Okamoto et al. proposes forming a diamond film by evaporating carbon onto the substrate by arc discharge at a carbon cathode and applying a negative bias voltage to the substrate so as to form a plasma glow discharge around the substrate. U.S. Pat. No. 4,830,702 to Singh et al. proposes a hollow cathode plasma assisted method and apparatus for forming diamond films. Unfortunately, such electrical discharge methods for forming diamond films often fail to produce high quality diamond films, or layers.
Microwave plasma enhanced CVD has also been used to form diamond films. In addition, techniques have been developed for enhancing the nucleation of diamond onto a silicon substrate, or other substrate, for the subsequent growth of a diamond film by a conventional growth process. For example, it is well known that the diamond nucleation density on a substrate may be increased several orders of magnitude by simply scratching or abrading the substrate prior to placing it into the conventional CVD growth chamber. Although the size and density of grown diamond particles can be controlled to some extent by the size and density of the scratches, each diamond particle still grows in a random orientation. In addition, the maximum density of diamond nuclei is also typically limited to less than about 10.sup.9 /cm.sup.2.
Other attempts have been made to more effectively seed the nucleation process, such as by spraying the substrate with diamond powder through an air brush, or by ultrasonically abrading the substrate surface. U.S. Pat. No. 4,925,701 to Jansen et al. proposes seeding a substrate with a diamond powder to enhance nucleation. Unfortunately, each of these types of preparation techniques has to be performed outside of the plasma CVD reaction chamber.
The scratching and seeding techniques, also fail to produce a surface which is sufficiently smooth to permit in-situ monitoring of the diamond growth rate. Therefore, ex-situ analysis is commonly used, such as cross-sectional scanning electron microscopy or profilometry. Such ex-situ analysis does not permit processing parameters to be controlled during the diamond growth process.
An article entitled Generation of Diamond Nuclei by Electric Field in Plasma Chemical Vapor Deposition, by Yugo et al. appearing in Applied Physics Letters, 58 (10) pp. 1036-1038, Mar. 11, 1991, proposes a predeposition of diamond nuclei on a silicon mirror surface prior to the conventional diamond CVD growth process. A high methane fraction (i.e., at least 5 percent) in the feed gas is taught by Yugo along with a negative electrical bias of 70 volts negative with respect to ground applied to the substrate for a time period of just several minutes.
The Yugo article also proposes that a balance must be struck between the biasing voltage and the methane content of the gas mixture. The authors of the Yugo article theorize that an excessive acceleration of the ions from a high voltage can destroy newly formed diamond nuclei. Yugo suggests that revaporization of the newly formed diamond nuclei should be suppressed by mitigating the ion impact by keeping the magnitude of the bias voltage low. Thus, in order to offset the low bias voltage, the degree of carbon over saturation, as determined by the methane percentage, should be increased. Yugo reported that diamond nuclei growth did not occur below 5% methane content and that high densities of nuclei occurred only above 10% methane. In addition, the absolute value of the biasing voltages were maintained below 200 volts negative with respect to ground to avoid revaporization from high energy impacting ions. The total time duration for the pretreatment was limited to between 2 to 15 minutes.
It will also be necessary to produce single crystal diamond thin films over relatively large areas of a substrate in order to fully utilize diamond as a semiconductor material from which to fabricate electronic devices. Homoepitaxial growth of diamond has been reported; however, diamond substrates of sufficient size to make the process economical are not currently available. The growth of heteroepitaxial, or textured diamond films comprising a plurality of locally heteroepitaxial diamond areas, therefore, becomes an important goal if the economical fabrication of diamond electronic devices, for example, is to become a reality.
Heteroepitaxial or textured growth has been reported on cubic-boron nitride (c-BN), nickel and silicon. C-BN has shown promise as a heteroepitaxial substrate for diamond due to its close lattice match and high surface energy. However, it is presently difficult to grow c-BN in large single crystal form. Recent results report that local epitaxial growth of diamond on nickel is attractive. Nickel has a close lattice match with diamond although its catalytic properties on the decomposition of hydrocarbons into sp.sup.2 bonded structures may make it difficult to inhibit the formation of graphite during diamond growth and nucleation. Furthermore, it is difficult to obtain diamond films which adhere well to nickel.
An article by Jeng et al. in Applied Physics Letters, 56 (20) p. 1968, (1990), reported limited texturing of diamond on silicon substrates having a semicrystalline silicon carbide surface conversion film thereon. The lattice match between .beta.-SiC (a=4.36 .ANG.) and diamond (a=3.57 .ANG.) is not extremely attractive; however, .beta.-SiC grows epitaxially on Si despite a 24% lattice mismatch.
Thus, there still exists a need to form high quality diamond films to take advantage of the many attractive properties of diamond. In addition, there also exists a need to improve heteroepitaxial or textured diamond films for many electronic applications.